FIG. 3 shows the structure of a typical solar cell in the conventional art. In one main surface side of a p-type Si substrate 21 made of single-crystal or polycrystalline Si having a thickness of about 0.25 mm is formed an emitter layer (an n+ layer) 22 having P or the like diffused therein in a depth of 0.1 μm to 0.5 μm, on which are formed an antireflection coating 23 made of Si3N4 or SiO2 for reducing a surface reflectance and a front surface electrode (i.e., a light-receiving surface electrode) 34 for taking out a current. In the other surface side (i.e., a back surface side) of the Si substrate is formed a BSF layer (a p+ layer) 25 having Al or the like diffused therein at a high concentration. Moreover, a back surface electrode 26 is formed on the back surface.
In fabricating a solar cell of this type, for the reasons of easiness and a low cost, the front surface electrode 34 is generally formed by printing and firing as follows. A conductive paste incorporating silver powder therein is generally used as a material for the front surface electrode. The conductive paste is applied by screen-printing or the like, followed by sintering at a high temperature inside of a firing furnace, thereby forming a front surface electrode. In such an electrode forming method, the conductive paste to be used normally includes mainly the silver powder, glass frit, an organic vehicle, and an organic solvent.
This light-receiving surface electrode needs to have a small occupation area and a low resistance in such a manner as not to shield light, and therefore, is required to be a thick electrode having a fine line width (i.e., a high aspect ratio). However, it is in principle difficult to form an electrode having a high aspect ratio by one printing in the screen-printing method. In view of this, as effective solving measures has been well known a multi-layered electrode structure obtained by printing electrode pastes on a solar cell substrate in superimposition a plurality of times by the screen-printing, thereby achieving a high aspect ratio (see, for example, Japanese Patent Application Laid-open No. H11-103084).
However, when the electrode having the high aspect ratio is formed by simply repeating the screen-printing and drying of the electrode pastes for a solar cell in the conventional art, like the above-described multi-layered electrode structure in the conventional art, there has arisen a problem that contraction of a silver particle during sintering sometimes induces a break, thereby making the electrode inoperative. Moreover, there has arisen problem that foaming, cracking, incomplete combustion of an organic binder contained in the conductive paste of a lower layer (a side of the substrate), or the like occurs during sintering the pastes for a multi-layered electrode, thereby degrading the characteristics of the electrode.